1. Field of the Invention
This invention relates in general to passive sensors, more particularly to passive acoustic sensors, and most particularly to passive acoustic sensors to detect sound emanating from under water.
2. Description of the Prior Art
There are many potential sources of acoustic sounds emanating from under water, such as in the ocean. For example, cetacean mammals, such as dolphins and whales, emit broadband, short duration clicks for echolocation and longer duration, narrow band, frequency modulated whistles for communication. Also, certain man-made objects, placed or released underwater, also project sounds at various frequencies.
The present passive method for identifying these types of sounds is to place hydrophones under the water and set these devices to record. After a certain period of time, these recordings are assembled into database systems. The database systems may be provided to researchers that study cetacean mammal migratory or behavioral patterns as well as users attempting to identify man-made sources projecting acoustic signatures underwater.
However, the use of such in-water devices is problematic because the devices themselves may disrupt the behavior of undersea life. Also, the ships that carry, place, and maintain the in-water devices produce significant noise that can also disrupt undersea life or interfere with acoustic signals emanating from underwater objects that the devices are attempting to identify.
Therefore, it is desired to provide a passive sensor to detect acoustic sounds emanating from under water that does not interfere with or startle the source of the acoustic sounds.